This invention relates to the preparation of linear, fiber forming, colorless polyesters. This invention further relates to a novel class of compounds that are useful as both transesterification and polycondensation catalysts for the preparation of colorless, fiber forming polyesters and to a method for preparing these compounds.
High molecular weight polyesters, particularly polyethylene terephthalate, are employed in large quantities for the preparation of textile fibers in addition to films for packaging and other uses. Polyesters of this type are produced on a commercial scale by reacting a dialkyl ester of a carboxylic acid with an alkylene glycol. The acid can be aliphatic or aromatic, however the acid used most frequently to prepare fiber forming polyesters is terephthalic acid, usually in the form of the dimethyl ester, and the most preferred glycol is ethylene glycol. Typically the terephthalic acid ester and glycol are reacted at temperatures from 140 to about 220.degree. C. under atmospheric pressure. The reaction is continued until substantially all of the methanol or other alcohol present in the initial dialkyl terephthalate has been removed from the reaction mixture. This stage of polyester preparation, known as ester interchange or transesterification, is conventionally conducted in the presence of a catalyst. Compounds which have heretofore been employed for this purpose include salts of the alkali metals and alkaline earth metals. The free metals themselves have also been employed, as have salts of manganese and zinc. The presence of these compounds or elements in catalytic amounts during the subsequent polycondensation stage is, in some instances, detrimental to certain properties, particularly color, of the final polyester. It is therefore common practice to add a sequestering agent such as a triester of phosphorous acid or other phosphorus compound for the purpose of inactivating the transesterification catalyst.
The second phase of polyester production is a polycondensation of the glycol ester formed during the ester interchange stage. The polycondensation is conventionally conducted at temperatures from 200.degree. to 300.degree. C., preferably from 270.degree. to 290.degree. C. under an inert atmosphere at pressures as low as can practically be achieved to minimize thermally induced degradation of the prepolymer. Removal of the by-product alkylene glycol as it is formed is considered essential to achieve the desired high molecular weight. The polycondensation step usually requires from 1 to 4 hours. The final polymer desirably exhibits an inherent viscosity greater than 0.5. Catalysts conventionally employed for the polycondensation step include compounds of antimony, titanium and tin. The catalytic activity of soluble antimony compounds such as antimony triacetate is considered outstanding, however many of these compounds are readily reduced to gray elemental antimony, particularly under the conditions employed for polycondensation. Since elemental antimony often imparts a gray color to the polymer, soluble antimony catalysts may not be useful for preparing colorless polymer in the absence of pigments or delusterants. Moreover, trivalent antimony compounds such as acid salts and alkoxides are readily hydrolyzed by the small amounts of water present in the atmosphere to form products that are insoluble even in catalytic amounts in the polycondensation reaction mixture.
The color and other properties of a polyester may be adversely affected by products of side reactions that occur during transesterification and polycondensation. Some polycondensation catalysts also catalyze formation of these by-products, which may include diethylene glycol and higher oligomers of ethylene glycol. These oligomers are particularly undesirable since they reduce the light stability of the polyester when incorporated into the polymer chain in place of ethylene glycol. In addition, polymer segments containing the aforementioned oligomers often exhibit a different affinity for dyes than "conventional" polymer segments, resulting in undesirable shade variations along the length of a fiber or between fibers prepared from different batches of the same polymer.
One objective of this invention is to define a class of catalysts that will yield colorless, high molecular weight polyesters. A second objective is to define a class of compounds that catalyze both ester interchange and polycondensation, do not require use of a sequestering agent and do not adversely affect the chemical or physical properties of the final polymer. Another objective is to decrease the minimum time intervals required for transesterification and polycondensation using prior art catalysts.
The foregoing objectives are achieved using a novel class of bimetallic compounds which catalyze both transesterification and polycondensation.